ABSTRACT: Members of the marine phytoplankton genus Phaeocystis (Prymnesiophyceae) produce large amounts of the intracellular osmolyte DMSP and they are known to also produce lyase enzymes that cleave DMSP into the biogeochemically important trace gas DMS. The functional characteristics of DMSP lyase activity in Phaeocystis spp. are not well known. We characterized DMSP lyase activity in extracts from 2 ecologically important species from this genus, the mesophile P. globosa (strain CCMP629) and the psychrophile P. antarctica (strain CCMP1374). Results from whole cell extracts showed that both algal species were potent producers of DMSP lyase, with Michaelis-Menten constant (K_m) and maximum reaction velocity (V_max) values of 1.77 mM and 17.3 nmol DMS min^-1 mg protein^-1, respectively, for P. globosa, and 2.31 mM and 28.2 nmol DMS min^-1 mg protein^-1, respectively, for P. antarctica. The optimal DMSP lyase activity was recorded at pH 4 and 30?C for P. globosa, and at pH 5 and 20?C for P. antarctica. The half-life of the DMSP lyase of P. globosa was 210 min at 25?C, which was longer than that of the P. antarctica enzyme (61.9 min). First-order kinetic analysis of DMSP lyase thermal denaturation demonstrated that the activation energy, free energy, enthalpy and entropy of denaturation in P. antarctica extracts were lower than for P. globosa extracts, confirming that the P. antarctica DMSP lyase was more thermolabile than the lyase from the temperate strain. Inhibitor tests with metals, a chelator (EDTA) and a serine binding agent (PMSF) suggested that the DMSP lyases from both Phaeocystis species were metalloenzymes with serine and sulfhydryl groups at the active site. The acidic pH optima for the Phaeocystis strains are consistent with findings from other Prymnesiophyceae, and we speculate that this may reflect adaptation to an acidic sub-cellular location for the DMSP lyase.